In an aerobic biological treatment of aqueous organic wastes by making use of the activity of aerobic microorganisms, such as the so-called activated sludge treatment, a large amount of "excess sludge" is formed, which is difficult to be disposed of. While heretofore such excess sludge has been disposed by depositing it in landfills, because landfills are becoming less available, it is now necessary to reduce the amount of excess sludge formed.
Japanese Patent Kokai JP-A-6-206088 discloses a technique for aerobic biological treatment of aqueous organic wastes in which a greater amount of the biosludge than the amount of that multiplied by an anabolic intake of the BOD components in the waste is extracted from the aerobic biological treatment system and is subjected to an ozone treatment before it is returned to the biological treatment system, the apparatus for which is illustrated in FIG. 4 in a flow diagram. In FIG. 4, numeral 1 represents the aerobic biological treatment system composed of an aeration tank 11 and a solid/liquid separation unit 12 and 2 is a modification treatment system. Over the bottom of the aeration tank 11 is disposed an air distributor 15, to which an air supply line 16 is connected. To the aeration tank 11, an aqueous waste supply line 17 and a biosludge return line 24 having a pump 23 are joined. The aeration tank 11 communicates to the solid/liquid separation unit 12 via a connection line 18. To the solid/liquid separation unit 12, a treated liquid line 21 and a separated sludge delivery line 22 are joined. The separated sludge delivery line 22 branches off into a sludge return line 24, a sludge extraction line 32 and an excess sludge delivery line 37.
The modification treatment system 2 functions to modify the biosludge to convert it into an easily biodegradable product by the ozonization and is provided with an ozonization vessel 31 connected, at its upper portion, with a sludge extraction line 32 guided from the solid/liquid separation unit 12 and a spent ozone discharge line 33 and, at its lower portion, with an ozone supply line 34 and an ozonized sludge delivery line 35. The ozonized sludge delivery line 35 communicates to the aeration tank 11. A pump 36 is provided in the sludge extraction line 32.
For effecting the aerobic biotreatment of an aqueous organic waste using the apparatus as shown in FIG. 4, the aqueous organic waste to be treated is guided first into the aeration tank 11 via the aqueous waste supply line 17 and is mixed therein with the return sludge from the sludge return line 24 and with the biosludge present in the aeration tank 11, while introducing therein atmospheric air through the air distributor 15 to effect an aerobic biological treatment. A part of the mixed liquor in the aeration tank 11 is guided to the solid/liquid separation unit 12 via the connection line 18 and is subjected to a solid/liquid separation into a separated liquid and a separated sludge. The separated liquid is exhausted out of the system via the treated liquid line 21, while a part of the separated sludge is returned as the return sludge to the aeration tank 11 via the sludge return line 24 and the other part is supplied as an extracted sludge to the ozonization vessel 31. If excess sludge occurs, it is discharged out of the system via the excess sludge delivery line 37.
In the modification treatment system 2, a part of the separated sludge is guided into the ozonization vessel 31 via the sludge extraction line 32. Ozone is supplied through ozone supply line 34 to cause it to contact with the extracted biosludge to effect ozonization. The living bacteria in the extracted biosludge will almost be destroyed thereby and the biosludge is modified into easily biodegradable BOD components of the aqueous waste. The ozonized sludge is recirculated from the ozonized sludge delivery line 35 to the aeration tank 11, where it is subjected to aerobic biotreatment. By subjecting the extracted biosludge to an ozone treatment and then to the aerobic biotreatment in this way, a reduction in the amount of excess sludge can be attained and even a complete elimination of occurrence of excess sludge may be realized when a greater amount of extracted biosludge than the amount of the biosludge multiplied in the aerobic biotreatment system is subjected to the ozone treatment in the modification system.
In the above-described prior technique, however, the average sludge retention time (SRT) in the aerobic biotreatment system with respect to the living bacteria becomes lower due to the annihilation of the bacteria upon the ozone treatment, as shown by the equation: EQU SRT of living bacteria=A/B (a)
in which A denotes the amount of biosludge retained in the aeration tank and B represents the amount of ozonized biosludge.
If, for example, the amount of excess sludge occurred is zero, about three times the amount of excess sludge obtained in a conventional activated sludge treatment should be extracted and subjected to the ozone treatment, so that the SRT of living bacteria herefor (below, the SRT of living bacteria is denoted by SRT') will be decreased to 1/3 of that in a conventional activated sludge treatment. Consequently, in the prior technique, insufficient biodegradation of organic matter may occur when the temperature of the aqueous waste is low, such as in winter, resulting in, for example, an instablility in the quality of the treated water and accumulation of undecomposed organic sludge in the flocs of biosludge which deteriorates the sedimenting property of the sludge in the aeration tank or in the solid/liquid separation tank.
Such problems may be avoided by installing further aeration units to reduce the load of the activated sludge, which, however, brings about increases in the installation site and construction investments accompanied by a greater economical restriction.